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Stainless steel: the story on HearLore | HearLore
Stainless steel
In 1913, a metallurgist named Harry Brearley was searching for a solution to a persistent problem in the British gun industry: barrels that rusted and degraded after repeated firing. While testing various chromium-iron alloys at the Brown-Firth research laboratory in Sheffield, England, he stumbled upon a material that did not rust. This discovery, initially called rustless steel, would eventually become known as stainless steel. The key to its resistance lay in the addition of chromium, which formed a passive film on the surface, protecting the underlying metal from oxidation. This film was self-healing, meaning that even if scratched, it could repair itself when exposed to oxygen. The alloy contained at least 10.5% chromium, a threshold that proved critical for its protective capabilities. Brearley's discovery was not immediately recognized as a revolution; it was only two years later, in January 1915, that The New York Times announced the development of this new alloy. The metal was later marketed under the Staybrite brand by Firth Vickers in England and was used for the new entrance canopy for the Savoy Hotel in London in 1929. The name stainless steel was coined by a local cutlery manufacturer with whom Brearley worked, and it eventually became the standard term worldwide, even influencing the Japanese language where Western cutlery is simply referred to as stainless.
The Chemistry Of Corrosion
The true power of stainless steel lies in its chemical composition, which allows it to resist corrosion in ways that ordinary steel cannot. Unlike carbon steel, which rusts readily when exposed to air and moisture, stainless steel contains sufficient chromium to undergo passivation. This process forms a microscopically thin inert surface film of chromium oxide by reacting with oxygen in the air and even the small amount of dissolved oxygen in water. This passive film prevents further corrosion by blocking oxygen diffusion to the steel surface, thus preventing corrosion from spreading into the bulk of the metal. The resistance of this film depends on the chemical composition of the stainless steel, chiefly the chromium content. To enhance this resistance, manufacturers can increase chromium content to more than 11%, add nickel to at least 8%, or add molybdenum, which also improves resistance to pitting corrosion. The addition of nitrogen also improves resistance to pitting corrosion and increases mechanical strength. There are numerous grades of stainless steel with varying chromium and molybdenum contents to suit the environment the alloy must endure. For example, Type 304 stainless steel is resistant to 3% acid at room temperature, while Type 316 is resistant to 3% acid up to 60 degrees Celsius and 20% acid at room temperature. This makes Type 304 stainless steel rarely used in contact with sulfuric acid, while Type 904L and Alloy 20 are resistant to sulfuric acid at even higher concentrations above room temperature.
Common questions
Who invented stainless steel and when was it discovered?
Harry Brearley invented stainless steel in 1913 while testing chromium-iron alloys at the Brown-Firth research laboratory in Sheffield, England. He discovered the material that did not rust, which was initially called rustless steel before becoming known as stainless steel.
What is the minimum chromium content required for stainless steel to resist corrosion?
Stainless steel must contain at least 10.5% chromium to form a self-healing passive film that protects the underlying metal from oxidation. This chromium threshold is critical for the alloy's protective capabilities and distinguishes it from ordinary carbon steel.
When was stainless steel first announced to the public and what was its first major commercial use?
The New York Times announced the development of stainless steel in January 1915, two years after its discovery. The metal was first used for the new entrance canopy of the Savoy Hotel in London in 1929 under the Staybrite brand by Firth Vickers.
What are the five families of stainless steel alloys and which is the largest?
Stainless steel is classified into five families: austenitic, ferritic, martensitic, duplex, and precipitation hardening. Austenitic stainless steel is the largest family, making up about two-thirds of all stainless steel production.
How much stainless steel was produced annually in the US in 1929 and what is its carbon footprint?
Over 25,000 tons of stainless steel were manufactured and sold in the US annually in 1929 before the Great Depression. The average carbon footprint of stainless steel is estimated to be 2.90 kg of CO2 per kg of stainless steel produced.
Is stainless steel considered safe for human health and what are the risks associated with welding it?
Stainless steel is generally considered biologically inert with no established connection between cookware and cancer. However, welding stainless steel produces carcinogenic fumes from cadmium oxides, nickel, and chromium, which were classified as a Group 1 carcinogen in 2017.
Stainless steel is classified into five different families of alloys, each having a distinct set of attributes. Four of the families are defined by their predominant crystalline structure: austenitic, ferritic, martensitic, and duplex alloys. The fifth family, precipitation hardening, is defined by the type of heat treatment used to develop its properties. Austenitic stainless steel is the largest family, making up about two-thirds of all stainless steel production. They have a face-centered cubic crystal structure and are not hardenable by heat treatment since they possess the same microstructure at all temperatures. Ferritic stainless steels have a body-centered cubic crystal structure, are magnetic, and are hardenable by cold working, but not by heat treating. They contain between 10.5% and 27% chromium with very little or no nickel, making them less expensive than austenitic stainless steels. Martensitic stainless steels have a body-centered tetragonal crystal structure, are magnetic, and are hardenable by heat treating and by cold working. They offer a wide range of properties and are used as stainless engineering steels, stainless tool steels, and creep-resistant steels. Duplex stainless steels have a mixed microstructure of austenite and ferrite, the ideal ratio being a 50:50 mix, though commercial alloys may have ratios of 40:60. They are characterized by higher chromium and molybdenum and lower nickel contents than austenitic stainless steels. Precipitation hardening stainless steels are characterized by the ability to be precipitation hardened to higher strength, with three types classified according to their crystalline structure: martensitic, semi-austenitic, and austenitic.
The Industrial Revolution
The history of stainless steel began long before its commercialization, with early scientific developments dating back to 1798 when chromium was first shown to the French Academy by Louis Vauquelin. In the early 1800s, British scientists James Stoddart, Michael Faraday, and Robert Mallet observed the resistance of chromium-iron alloys to oxidizing agents. Robert Bunsen discovered chromium's resistance to strong acids, and the corrosion resistance of iron-chromium alloys may have been first recognized in 1821 by Pierre Berthier, who noted their resistance against attack by some acids and suggested their use in cutlery. In the 1840s, both Britain's Sheffield steelmakers and then Krupp of Germany were producing chromium steel with the latter employing it for cannons in the 1850s. In 1861, Robert Forester Mushet took out a patent on chromium steel in Britain. These events led to the first American production of chromium-containing steel by J. Baur of the Chrome Steel Works of Brooklyn for the construction of bridges. A US patent for the product was issued in 1869. This was followed with recognition of the corrosion resistance of chromium alloys by Englishmen John T. Woods and John Clark, who noted ranges of chromium from 5, 30%, with added tungsten and medium carbon. They pursued the commercial value of the innovation via a British patent for Weather-Resistant Alloys. Scientists researching steel corrosion in the second half of the 19th century didn't pay attention to the amount of carbon in the alloyed steels they were testing until in 1898 Adolphe Carnot and E. Goutal noted that chromium steels better resist to oxidation with acids the less carbon they contain. Also in the late 1890s, German chemist Hans Goldschmidt developed an aluminothermic process for producing carbon-free chromium. Between 1904 and 1911, several researchers, particularly Leon Guillet of France, prepared alloys that would later be considered stainless steel. In 1908, the Essen firm Friedrich Krupp Germaniawerft built the 366-ton sailing yacht Germania featuring a chrome-nickel steel hull, in Germany. In 1911, Philip Monnartz reported on the relationship between chromium content and corrosion resistance. On the 17th of October 1912, Krupp engineers Benno Strauss and Eduard Maurer patented as Nirosta the austenitic stainless steel that became known as 18/8 or AISI type 304.
The Global Market
The global production of stainless steel has grown significantly since its inception, with world stainless steel production figures published yearly by the International Stainless Steel Forum. Of the EU production figures, Italy, Belgium, and Spain were notable, while Canada and Mexico produced none. China, Japan, South Korea, Taiwan, India, the US, and Indonesia were large producers while Russia reported little production. In 1929, before the Great Depression, over 25,000 tons of stainless steel were manufactured and sold in the US annually. Major technological advances in the 1950s and 1960s allowed the production of large tonnages at an affordable cost, including the AOD process for the removal of carbon and sulfur, continuous casting and hot strip rolling, the Z-Mill or Sendzimir cold rolling mill, and the Creusot-Loire Uddeholm processes which use steam instead of some or all of the argon. The breakdown of production by stainless steels families in 2017 showed that austenitic stainless steels Cr-Ni made up 54%, austenitic stainless steels Cr-Mn made up 21%, and ferritic and martensitic stainless steels made up 23%. The average carbon footprint of stainless steel is estimated to be 2.90 kg of CO2 per kg of stainless steel produced, of which 1.92 kg are emissions from raw materials, 0.54 kg from electricity and steam, and 0.44 kg are direct emissions. Stainless steel is 100% recyclable, with an average stainless steel object composed of about 60% recycled material of which approximately 40% originates from end-of-life products, while the remaining 60% comes from manufacturing processes. The per capita stock of stainless steel in use in society is 180 kg in more developed countries and 10 kg in less-developed countries.
The Applications Of Steel
The future of stainless steel lies in its ability to adapt to new challenges and technologies. Nanoscale stainless steel nanoparticles have been produced in the laboratory, and these may have applications as additives for high-performance applications. For example, sulfurization, phosphorization, and nitridation treatments to produce nanoscale stainless steel based catalysts could enhance the electrocatalytic performance of stainless steel for water splitting. The health effects of stainless steel are also a subject of extensive research, with some studies indicating a probable increased risk of cancer from inhaling fumes while welding stainless steel. Stainless steel welding is suspected of producing carcinogenic fumes from cadmium oxides, nickel, and chromium. In 2017, all types of welding fumes were classified as a Group 1 carcinogen. However, stainless steel is generally considered to be biologically inert, and no connection between stainless steel cookware and cancer has been established. The life cycle cost calculations are used to select the design and the materials that will lead to the lowest cost over the whole life of a project, such as a building or a bridge. The higher acquisition cost of stainless steel components are often offset by improvements in operating and maintenance costs, reduced loss of production costs, and the higher resale value of stainless steel components. The average carbon footprint of stainless steel is estimated to be 2.90 kg of CO2 per kg of stainless steel produced, of which 1.92 kg are emissions from raw materials, 0.54 kg from electricity and steam, and 0.44 kg are direct emissions. Stainless steel produced in countries that use cleaner sources of electricity will have a lower carbon footprint. Ferritics without Ni will have a lower CO2 footprint than
The Future Of Steel
austenitics with 8% Ni or more. The stainless steel cycle starts with carbon steel scrap, primary metals, and slag, and the next step is the production of hot-rolled and cold-finished steel products in steel mills. Some scrap is produced, which is directly reused in the melting shop. The manufacturing of components is the third step. Some scrap is produced and enters the recycling loop. Assembly of final goods and their use does not generate any material loss. The fourth step is the collection of stainless steel for recycling at the end of life of the goods, such as kitchenware, pulp and paper plants, or automotive parts. This is where it is most difficult to get stainless steel to enter the recycling loop, as shown in the table below. Estimates of collection for recycling by sector show that building and infrastructure has a 17% collection rate, transportation has a 21% collection rate, industrial machinery has a 29% collection rate, household appliances and electronics have a 10% collection rate, and metal goods have a 23% collection rate.